How do you cut off part of a mesh you don't want?
Switch to Edit Mode.

You can do this by going to the Tab (Don't know what to call it) near the bottom, most likely called Object Mode and clicking it followed by Edit Mode. You can also press the Tab key, which is the shortcut.

All of the vertices should be selected, which you can tell if they are orange. We don't want all of the vertices selected. Press A to deselect all the vertices, it should go from orange to black.

Now you need to select the area you would like to delete. Press B and drag over the area desired. Make sure to rotate around the model since it won't select the vertices on the other side (and I don't know how to fix that but it should work either way)

After the vertices of the desired area has been selected press X and then click 'Vertices'

Hope this helps.